I thought Bitcoin is too outdated and threatened by better projects to still remain as the no. 1 cryptocurrency. Now I take it all back.

Reasons why I had to reconsider everything were:

Ultimate decentralization: Because there's no head entity nothing much can be manipulated. We as a whole decide what happens and what doesn't, but there's no creator (who we know of) controlling Bitcoin. It's just there, working like a perpetual motion machine.
Extensive flexibility: Even though Bitcoin doesn't originally support Dapps or fast transactions, 'extensions' like Lightning network and Stacks Dapps have helped Bitcoin to grow and thrive.
First mover advantage: This makes other projects often follow Bitcoin in price action.
Seen as a store of value and the face of crypto: People expect no more from Bitcoin usually. They're just content with what it is and what it does. Some don't even recall the word "cryptocurrency" but they do remember hearing "Bitcoin" and that helps with adoption.
High volatility, but also generally stable: This one is self-explanatory. It's usually volatile within a range which helps with TA, but doesn't crash too hard. Not as hard as other projects.

I believe Ethereum is a huge competitor, though I'm glad to be a part of a market in which they coexist.